Multiple other sector information combining for power control in a wireless communication system

ABSTRACT

Techniques for adjusting transmit power to mitigate both intra-sector interference to a serving base station and inter-sector interference to neighbor base stations are described. This may be done by combining interference information from multiple base stations.

CLAIM OF PRIORITY UNDER 35 U.S.C. §120

The present application for patent is a Continuation and claims priorityto patent application Ser. No. 11/376,772 entitled “Multiple othersector information combining for power control in a wirelesscommunication system” filed Mar. 15, 2006, and assigned to the assigneehereof and hereby expressly incorporated by reference herein.

BACKGROUND

I. Field

The present invention relates generally to communication, and morespecifically to use of information from multiple sectors for powercontrol in a wireless terminal

II. Background

A wireless multiple-access communication system can simultaneouslysupport communication for multiple wireless terminals. Each terminalcommunicates with one or more sectors via transmissions on the forwardand reverse links. The forward link (or downlink) refers to thecommunication link from the sectors to the terminals, and the reverselink (or uplink) refers to the communication link from the terminals tothe sectors.

Multiple terminals may simultaneously transmit on the reverse link bymultiplexing their transmissions to be orthogonal to one another. Themultiplexing attempts to achieve orthogonality among the multiplereverse link transmissions in time, frequency, and/or code domain.Complete orthogonality, if achieved, results in the transmission fromeach terminal not interfering with the transmissions from otherterminals at a receiving sector. However, complete orthogonality amongthe transmissions from different terminals is often not realized due tochannel conditions, receiver imperfections, and so on. The loss inorthogonality results in each terminal causing some amounts ofinterference to other terminals communicating with the same sector.Furthermore, the transmissions from terminals communicating withdifferent sectors are typically not orthogonal to one another. Thus,each terminal may also cause interference to terminals communicatingwith nearby sectors. The performance of each terminal is then degradedby the interference from all other terminals in the system.

There is therefore a need in the art for techniques to mitigate theeffects of interference so that improved performance may be achieved.

SUMMARY

Techniques for controlling transmit power for a data transmission from awireless terminal in a manner to mitigate both “intra-sector”interference and “inter-sector” interference are described herein. Thetransmit power is adjusted such that the amount of intra-sectorinterference the terminal may cause to a “serving” sector and the amountof inter-sector interference the terminal may cause to “neighbor”sectors are both maintained within acceptable levels. (The terms inquote are described below.) The amount of inter-sector interference theterminal may cause may be roughly estimated based on (1) the totalinterference observed by each neighbor sector, (2) channel gains for theserving and neighbor sectors, (3) the current transmit power level usedby the terminal, and (4) possibly other parameters. Each sector maybroadcast a report (e.g., a value) indicative of the total interferenceobserved by that sector. The channel gain for each sector may beestimated based on a pilot received from the sector. The transmit powermay be adjusted in a probabilistic manner, a deterministic manner, orsome other manner based on combining the interference reports from anumber of sectors for a single transmit power adjustment.

In general, the transmit power may be decreased if high interference isobserved by neighbor sectors and increased if low interference isobserved. The transmit power may also be adjusted by a larger amountand/or more frequently if (1) the terminal is located closer to aneighbor sector observing high interference and/or (2) the currenttransmit power level is higher. The transmit power may be adjusted by asmaller amount and/or less frequently if (1) the terminal is locatedcloser to the serving sector and/or (2) the current transmit power levelis lower. The intra-sector interference caused by the terminal ismaintained within an acceptable level by limiting the received signalquality (SNR) for the data transmission to be within a range ofallowable SNRs.

Various aspects and embodiments of the invention are described infurther detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and nature of the present invention will become moreapparent from the detailed description set forth below when taken inconjunction with the drawings in which like reference charactersidentify correspondingly throughout and wherein:

FIG. 1 shows a wireless multiple-access communication system;

FIG. 2 shows frequency hopping on a time-frequency plane;

FIG. 3 shows a method of adjusting transmit power by combininginterference indications from multiple sectors;

FIG. 4A shows a process for adjusting transmit power in a probabilisticmanner;

FIG. 4B shows a process for adjusting transmit power in a deterministicmanner;

FIG. 5 shows a power control mechanism for a data channel;

FIG. 6 shows a power control mechanism for a control channel; and

FIG. 7 shows a terminal, a serving sector, and a neighbor sector.

DETAILED DESCRIPTION

The word “exemplary” is used herein to mean “serving as an example,instance, or illustration.” Any embodiment or design described herein as“exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments or designs.

FIG. 1 shows a wireless multiple-access communication system 100. System100 includes a number of base stations 110 that support communicationfor a number of wireless terminals 120. Terminals 120 are typicallydispersed throughout the system, and each terminal may be fixed ormobile. A terminal may also be referred to as a mobile station, a userequipment (UE), a wireless communication device, or some otherterminology. A base station is a fixed station used for communicatingwith the terminals and may also be referred to as an access point, aNode B, or some other terminology. A system controller 130 couples tobase stations 110, provides coordination and control for these basestations, and further controls the routing of data for the terminalsserved by these base stations.

Each base station 110 provides communication coverage for a respectivegeographic area 102. A base station and/or its coverage area may bereferred to as a “cell”, depending on the context in which the term isused. To increase capacity, the coverage area of each base station maybe partitioned into multiple (e.g., three) sectors 104. Each sector isserved by a base transceiver subsystem (BTS). The term “sector” canrefer to a BTS and/or its coverage area, depending on the context inwhich the term is used. For a sectorized cell, the base station for thatcell typically includes the BTSs for all sectors of that cell. Forsimplicity, in the following description, the term “base station” isused generically for both a fixed station that serves a cell and a fixedstation that serves a sector. A “serving” base station or “serving”sector is one with which a terminal communicates. A “neighbor” basestation or “neighbor” sector is one with which the terminal is not incommunication. For simplicity, the following description assumes thateach terminal communicates with one serving base station, although thisis not a required limitation for the techniques described herein.

The power control techniques described herein may be used for variouswireless communication systems. For example, these techniques may beused for a Time Division Multiple Access (TDMA) system, a FrequencyDivision Multiple Access (FDMA) system, an orthogonal frequency divisionmultiple access (OFDMA) system, and so on. A TDMA system uses timedivision multiplexing (TDM), and transmissions for different terminalsare orthogonalized by transmitting in different time intervals. An FDMAsystem uses frequency division multiplexing (FDM), and transmissions fordifferent terminals are orthogonalized by transmitting in differentfrequency sub-carriers. TDMA and FDMA systems may also use code divisionmultiplexing (CDM). In this case, transmissions for multiple terminalsmay be orthogonalized using different orthogonal (e.g., Walsh) codeseven though they are sent in the same time interval or frequencysub-carrier. An OFDMA system utilizes orthogonal frequency divisionmultiplexing (OFDM), which effectively partitions the overall systembandwidth into a number of (N) orthogonal frequency sub-carriers. Thesesub-carriers are also referred to as tones, bins, frequency channels,and so on. Each sub-carrier may be modulated with data. An OFDMA systemmay use any combination of time, frequency, and/or code divisionmultiplexing. For clarity, the power control techniques are describedbelow for an OFDMA system.

FIG. 2 illustrates frequency hopping (FH) on a time-frequency plane 200for an OFDMA system. With frequency hopping, each traffic channel isassociated with a specific FH sequence that indicates the particularsub-carrier(s) to use for that traffic channel in each time interval.The FH sequences for different traffic channels in each sector areorthogonal to one another so that no two traffic channels use the samesub-carrier in any time interval. The FH sequences for each sector arealso pseudo-random with respect to the FH sequences for nearby sectors.Interference between two traffic channels in two sectors occurs wheneverthese two traffic channels use the same sub-carrier in the same timeinterval. However, the inter-sector interference is randomized due tothe pseudo-random nature of the FH sequences used for different sectors.

Data channels may be assigned to active terminals such that each datachannel is used by only one terminal at any given time. To conservesystem resources, control channels may be shared among multipleterminals using, e.g., code division multiplexing. If the data channelsare orthogonally multiplexed only in frequency and time (and not code),then they are less susceptible to loss in orthogonality due to channelconditions and receiver imperfections than the control channels.

The data channels thus have several key characteristics that arepertinent for power control. First, intra-cell interference on the datachannels is minimal because of the orthogonal multiplexing in frequencyand time. Second, inter-cell interference is randomized because nearbysectors use different FH sequences. The amount of inter-cellinterference caused by a given terminal is determined by (1) thetransmit power level used by that terminal and (2) the location of theterminal relative to the neighbor sectors.

For the data channels, power control may be performed such that eachterminal is allowed to transmit at a power level that is as high aspossible while keeping intra-cell and inter-cell interference to withinacceptable levels. A terminal located closer to its serving sector maybe allowed to transmit at a higher power level since this terminal willlikely cause less interference to neighbor sectors. Conversely, aterminal located farther away from its serving sector and toward asector edge may be allowed to transmit at a lower power level since thisterminal may cause more interference to neighbor sectors. Controllingtransmit power in this manner can potentially reduce the totalinterference observed by each sector while allowing “qualified”terminals to achieve higher SNRs and thus higher data rates.

Power control for the data channels may be performed in various mannersto attain the goals noted above. For clarity, a specific embodiment ofpower control is described below. For this embodiment, the transmitpower for a data channel for a given terminal may be expressed as:

P _(dch)(n)=P _(ref)(n)+ΔP(n),  Eq (1)

where P_(dcg)(n) is the transmit power for the data channel for updateinterval n;

P_(ref)(n) is a reference power level for update interval n; and

ΔP(n) is a transmit power delta for update interval n.

The power levels P_(dch)(n) and P_(ref)(n) and the transmit power deltaΔP(n) are given in units of decibels (dB).

The reference power level is the amount of transmit power needed toachieve a target signal quality for a designated transmission (e.g., ona control channel). Signal quality (denoted as SNR) may be quantified bya signal-to-noise ratio, a signal-to-noise-and-interference ratio, andso on. The reference power level and the target SNR may be adjusted by apower control mechanism to achieve a desired level of performance forthe designated transmission, as described below. If the reference powerlevel can achieve the target SNR, then the received SNR for the datachannel may be estimated as:

SNR _(dch)(n)=SNR _(target) +ΔP(n).  Eq (2)

Equation (2) assumes that the data channel and the control channel havesimilar interference statistics. This is the case, for example, if thecontrol and data channels from different sectors may interfere with oneanother. The reference power level may be determined as described below.

The transmit power for the data channel may be set based on variousfactors such as (1) the amount of inter-sector interference the terminalmay be causing to other terminals in neighbor sectors, (2) the amount ofintra-sector interference the terminal may be causing to other terminalsin the same sector, (3) the maximum power level allowed for theterminal, and (4) possibly other factors. Each of these factors isdescribed below.

The amount of inter-sector interference each terminal may cause may bedetermined in various manners. For example, the amount of inter-sectorinterference caused by each terminal may be directly estimated by eachneighbor sector and sent to the terminal, which may then adjust itstransmit power accordingly based upon the combination of theinter-sector interference estimates transmitted. This individualizedinterference reporting may require extensive overhead signaling. Forsimplicity, the amount of inter-sector interference each terminal maycause may be roughly estimated based on (1) the total interferenceobserved by each neighbor sector, (2) the channel gains for the servingand neighbor sectors, and (3) the transmit power level used by theterminal Quantities (1) and (2) are described below.

Each sector can estimate the total or average amount of interferenceobserved by that sector. This may be achieved by estimating theinterference power on each sub-carrier and computing an averageinterference power based on the interference power estimates for theindividual sub-carriers. The average interference power may be obtainedusing various averaging techniques such as, for example, arithmeticaveraging, geometric averaging, SNR-based averaging, and so on.

In certain aspects, arithmetic averaging of the interference at thesector may be utilized. In other aspects, geometric averaging may beutilized. In other aspects, SNR type averaging may be utilized.Different approaches and techniques of averaging are depicted anddisclosed in co-pending U.S. patent application Ser. No. 10/897,463,which is incorporate by reference in its entirety.

Regardless of which averaging technique is used, each sector may filterthe interference power estimates and/or the average interference powerover multiple time intervals to improve the quality of the interferencemeasurement. The filtering may be achieved with a finite impulseresponse (FIR) filter, an infinite impulses response (IIR) filter, orsome other types of filter known in the art. The term “interference” maythus refer to filtered or unfiltered interference in the descriptionherein.

Each sector may broadcast its interference measurements for use byterminals in other sectors. The interference measurements may bebroadcast in various manners. In one embodiment, the averageinterference power (or the “measured” interference) is quantized to apredetermined number of bits, which are then sent via a broadcastchannel. In another embodiment, the measured interference is broadcastusing a single bit that indicates whether the measured interference isgreater than or below a nominal interference threshold. In yet anotherembodiment, the measured interference is broadcast using two bits. Onebit indicates the measured interference relative to the nominalinterference threshold. The other bit may be used as a distress/panicbit that indicates whether the measured interference exceeds a highinterference threshold. The interference measurements may also be sentin other manners.

For simplicity, the following description assumes the use of a singleother-sector interference (OSI) bit to provide interference information.Each sector may set its OSI value (OSIB) as follows: ‘0’ ifI_(meas,m)(n)<I_(target); ‘1’ if I_(meas,m)(n)≧I_(target); and ifI_(meas,m)(n)≧I_(target)+N, where I_(target) is the nominal interferencethreshold, I_(meas,m) is the measured interference, and N is some upperbound threshold indicating an upper bound threshold indicative ofexcessive interference.

Alternatively, each sector may obtain a measuredinterference-over-thermal (IOT), which is a ratio of the totalinterference power observed by the sector to the thermal noise power.The total interference power may be computed as described above. Thethermal noise power may be estimated by turning off the transmitter andmeasuring the noise at the receiver. A specific operating point may beselected for the system and denoted as IOT_(target). A higher operatingpoint allows the terminals to use higher transmit powers (on average)for the data channels. However, a very high operating point may not bedesirable since the system can become interference limited, which is asituation whereby an increase in transmit power does not translate to anincrease in received SNR. Furthermore, a very high operating pointincreases the likelihood of system instability. In any case, each sectormay set its OSI value as follows: ‘0’ if IOT_(meas,m)(n)<IOT_(target);‘1’ if IOT_(meas,m)(n) IOT_(target); and ‘2’ ifIOT_(meas,m)(n)≧IOT_(target)+N, where IOT_(meas,m)(n) is the measuredIOT for sector m in time interval n and N is some upper bound thresholdindicative of excessive interference.

For both cases, the OSI value may be used for power control as describedbelow. It should be noted that the OSI value, may have any desired sizeand have more, or less, than three states.

Each terminal can estimate the channel gain (or propagation path gain)for each sector that may receive a reverse link transmission from theterminal. The channel gain for each sector may be estimated byprocessing a pilot received from the sector via the forward link,estimating the received pilot strength/power, and filtering pilotstrength estimates over time (e.g., with a filter having a time constantof several hundred milli-seconds) to remove the effects of fast fadingand so on. If all sectors transmit their pilots at the same power level,then the received pilot strength for each sector is indicative of thechannel gain between that sector and the terminal. The terminal may forma channel gain ratio vector, G, as follows:

$\begin{matrix}{{\underset{\_}{G} = \lbrack {{r_{1}(n)}{r_{2}(n)}\mspace{14mu} \ldots \mspace{14mu} {r_{M}(n)}} \rbrack},{where}} & {{Eq}\mspace{14mu} (3)} \\{{{r_{i}(n)} = {\frac{g_{s}(n)}{g_{ni}(n)}\frac{p_{ni}(n)}{p_{s}(n)}}},} & {{Eq}\mspace{14mu} (4)}\end{matrix}$

-   g_(s)(n) is the channel gain between the terminal and the serving    sector;-   g_(ni)(n) is the channel gain between the terminal and neighbor    sector i;-   p_(s)(n) is the relative power of a signal, e.g. pilot, originating    from the serving sector and terminating at the terminal;-   p_(ni)(n) is relative power of a signal, e.g. pilot, originating    from the neighbor sector i and terminating at the terminal; and-   r_(i)(n) is the channel gain ratio for neighbor sector i.

Since distance is inversely related to channel gain, the channel gainratio g_(s)(n)/g_(ni)(n) may be viewed as a “relative distance” that isindicative of the distance to a neighbor sector i relative to thedistance to the serving sector. In general, the channel gain ratio for aneighbor sector, r_(i)(n), decreases as the terminal moves toward thesector edge and increases as the terminal moves closer to the servingsector. The channel gain ratio vector, G, may be used for power controlas described below.

Although the data channels for each sector are multiplexed such thatthey are orthogonal to one another, some loss in orthogonality mayresult from inter-carrier interference (ICI), inter-symbol interference(ISI), and so on. This loss of orthogonality causes intra-sectorinterference. To mitigate intra-sector interference, the transmit powerof each terminal may be controlled such that the amount of intra-sectorinterference that this terminal may cause to other terminals in the samesector is maintained within an acceptable level. This may be achieved,for example, by requiring the received SNR for the data channel for eachterminal to be within a predetermined SNR range, as follows:

SNR_(dch)(n)ε[SNR_(min),SNR_(max)],  Eq (5)

where SNR_(min) is the minimum received SNR allowable for a datachannel; and

SNR_(max) is the maximum received SNR allowable for a data channel.

The minimum received SNR ensures that all terminals, especially thoselocated near the sector edge, can achieve a minimum level ofperformance. Without such a constraint, terminals located near thesector edge may be forced to transmit at an extremely low power level,since they often contribute a significant amount of inter-sectorinterference.

If the received SNRs for the data channels for all terminals areconstrained to be within the range [SNR_(min), SNR_(max)] then theamount of intra-sector interference caused by each terminal due to aloss of orthogonality may be assumed to be within the acceptable level.By limiting the received SNRs to be within this SNR range, there can)still be as much as (SNR_(max)−SNR_(min)) dB difference in receivedpower spectral density between adjacent sub-carriers (assuming thatsimilar amounts of inter-sector interference are observed on thesub-carriers, which is true, e.g., if the control and data channels hoprandomly so that the control and data channels from different sectorsmay collide with one another). A small SNR range improves the robustnessof the system in the presence of ICI and ISI. An SNR range of 10 dB hasbeen found to provide good performance in most operating scenarios.Other SNR ranges may also be used.

If the transmit power for the data channel is determined as shown inequation (1), then the received SNR for the data channel may bemaintained within the range of [SNR_(min), SNR_(max)] by constrainingthe transmit power delta, ΔP(n), to be within a corresponding range, asfollows:

ΔP(n)ε[ΔP_(min),ΔP_(max)],  Eq (6)

where ΔP_(min) is the minimum transmit power delta allowable for a datachannel, and

ΔP_(max) is the maximum transmit power delta allowable for a datachannel.

In particular, and ΔP_(min)=SNR_(min) −SNR _(target) andΔP_(max)=SNR_(max)−SNR_(target). In another embodiment, the transmitpower P_(dch)(n) may be constrained to be within a range that isdetermined, for example, based on the received signal power for the datachannel. This embodiment may be used, for example, if interference poweris statistically different among the sub-carriers.

The transmit power for the data channel for each terminal may then beadjusted based on the following parameters:

The OSI value broadcast by each sector;

The channel gain ratio vector, G, computed by the terminal;

The range of received SNRs allowable for the data channels, [SNR_(min),SNR_(max)], or equivalently the range of allowable transmit powerdeltas, [ΔP_(min), ΔP_(max)]; and

The maximum power level, P_(max), allowed for the terminal, which mayset by the system or the power amplifier within the terminal

Parameters 1) and 2) relate to the inter-sector interference caused bythe terminal Parameter 3) relates to the intra-sector interferencecaused by the terminal

In general, a terminal located close to a neighbor sector that reportshigh interference may transmit with a lower transmit power delta so thatits received SNR is closer to SNR_(min). Conversely, a terminal locatedclose to its serving sector may transmit with a higher transmit powerdelta so that its received SNR is closer to SNR_(max). A gradation ofreceived SNRs may be observed for the terminals in the system based ontheir proximity to the serving sectors. A scheduler at each sector cantake advantage of the distribution of received SNRs to achieve highthroughput while ensuring fairness for the terminals.

The transmit power for the data channel may be adjusted in variousmanners based on the four parameters noted above. The power controlmechanism does not need to maintain equal SNR for all terminals,especially in an orthogonal system like an OFDMA system, where terminalscloser to a sector may transmit at higher power levels without causingmuch problem to other terminals. For clarity, a specific embodiment foradjusting transmit power is described below. For this embodiment, eachterminal monitors the OSI values broadcast by neighbor sectors and thencombines the OSI values from multiple neighbor sectors to determinewhether to increase, decrease, or maintain its reverse link transmitpower.

An algorithm which adjusts the terminal transmit power based on OSIvalues from M neighbor sectors should be provided such that the OSIB ofa neighbor sector that has a lower channel gain should have more effecton the power adjustment compared to the OSIB of a neighbor sector whichhas a higher channel gain. Further, if there is only one neighborsector, the algorithm should be equivalent to using only the OSIB ofthat sector. Additionally, if there are two neighbor sectors which haveapproximately the same channel gain, there should be a power decrease ifany sector indicates interference levels above its threshold, e.g.OSIB=1, or 2, from any sector. That is, if any of the “close” neighborsectors experience excessive interference, then the terminal shoulddecrease its power to help the neighbor sector to decrease itsinterference.

The combined OSI value thus determines the direction in which to adjustthe transmit power. The amount of transmit power adjustment for eachterminal may be dependent on (1) the current transmit power level (orthe current transmit power delta) of the terminal and (2) the channelgain ratio for the sectors from which the OSI values were combined. Anexemplary, method is depicted in FIG. 3.

FIG. 3 shows a method of adjusting transmit power by combininginterference indications from multiple sectors. Initially, adetermination as to a number of sectors for which OSI values detected,block 210. If the number is zero, then the maximum available value forΔP(n) may be utilized, block 215. If the number is one, then a poweradjustment algorithm may be utilized the single OSI value, block 220.Various, exemplary approaches, are depicted and discussed with respectto FIGS. 4A and 4B. However, other approaches and techniques may beutilized.

If the number is two or more, a channel gain ratio is determined foreach sector to be utilized for the power adjustment, block 225. Thesemay be for all of the sectors from which the terminal can receivesignals, e.g. pilots, or a subset of these sectors. The determinationmay be based upon the following:

$\begin{matrix}{{ChanDiff}_{i} = {\frac{{RxPower}_{{RL},{SS}}}{{TransmitPower}_{{RL},{SS}}} \times \frac{{TransmitPower}_{i}}{{RxPower}_{i}}}} & {{Eq}.\mspace{14mu} (7)}\end{matrix}$

where RxPower_(RL,SS) is the power of pilots received at the terminalfor the reverse link serving sector;

TransmitPower_(RL,SS) is the power of pilots transmitted from thereverse link serving sector, which is a system parameter;

RxPower_(i) is the power of pilots received at the terminal for the ithsector; and

TransmitPower_(i) is the power of pilots transmitted from the ithsector; sector, which is a system parameter.

It should be noted that the power of the pilots transmitted, may beprovided in a message header or may be constant throughout the system.For example, if the pilots are acquisition pilots, then the power may bethe maximum power allowable at the sector for some number of symbolperiods.

The terminal then determines a threshold for each OSI value received,block 230. The threshold for each sectors OSI value may be determined asfollows:

$\begin{matrix}{{Threshold}_{i} = \{ \begin{matrix}{\max \begin{Bmatrix}{{UpDecisionThresholdMin},} \\{( {1 - a} )b_{i}}\end{Bmatrix}} & {{{if}\mspace{14mu} {OSI}_{i}} = 0} \\{\max \{ \begin{matrix}{{UpDecisionThresholdMin},} \\{( {1 - a} )b_{i}}\end{matrix} } & {{{if}\mspace{14mu} {OSI}_{i}} = 1} \\1 & {{{if}\mspace{14mu} {OSI}_{i}} = 2}\end{matrix} } & {{Eq}.\mspace{14mu} (8)}\end{matrix}$

where UpDecisionThresholdMin and DownDecisionThresholdMin arepredetermined system parameters which may be fixed or may be updatedduring any communication session. The variables a and b_(i) may bedetermined as follows:

$\begin{matrix}{{a_{i} = \frac{\begin{matrix}{{\min \{ {{RDCHGain},\; {RDCHGainMax}} \}} -} \\{RDCHGainMin}\end{matrix}}{{RDCHGainMax} - {RDCHGainMin}}},{and}} & {{Eq}.\mspace{14mu} (9)} \\{{b_{i} = \frac{\begin{matrix}{{\min \{ {{ChanDiff}_{i},{ChanDiffMax}} \}} -} \\{ChanDiffMin}\end{matrix}}{{ChanDiffMax} - {ChanDiffMin}}},} & {{Eq}.\mspace{14mu} (10)}\end{matrix}$

where RDCHGainMax is the maximum gain, RDCHGainMin is the minimum gain,ChanDiffMax is the maximum channel gain, and ChanDiffMin is the minimumchannel gain. These are predetermined system parameters which may befixed or may be updated during any communication session.

The terminal may then determine whether each threshold indicates thatthe power should be increase, decreased, or maintained for that OSIvalue, block 235. This determination may be made as follows:

$\begin{matrix}{{Decision}_{i} = \{ \begin{matrix}{UpDecisionValue} & \begin{matrix}{{{if}\mspace{14mu} x_{i}} \leq {DecisionThreshold}_{i}} \\{{{and}\mspace{14mu} {OSI}_{i}} = 0}\end{matrix} \\{- {DownDecisionValue}} & \begin{matrix}{{{if}\mspace{14mu} x_{i}} \leq {DecisionThreshold}_{i}} \\{{{and}\mspace{14mu} {OSI}_{i}} = {1\mspace{14mu} {or}\mspace{14mu} 2}}\end{matrix} \\0 & {otherwise}\end{matrix} } & {{Eq}.\mspace{14mu} (11)}\end{matrix}$

where 0≦x_(i)≦1, UpDecisionValue, and DownDecisionValue arepredetermined system parameters which may be fixed or may be updatedduring any communication session.

The terminal then combines the channel gains and indications of poweradjustments, based upon some weighting, to generate a weighted decision,block 240. The weighted decision may be determined as shown below:

$\begin{matrix}{D_{w} = \frac{\sum\limits_{i = 1}^{OSIMonitorSetSize}{\frac{1}{{ChanDiff}_{i}}{Decision}_{i}}}{\sum\limits_{i = 1}^{OSIMonitorSetSize}\frac{1}{{ChanDiff}_{i}}}} & {{Eq}.\mspace{14mu} (12)}\end{matrix}$

Where ChanDiff_(i) is the channel gain for each terminal, OSIMonitor SetSize the number of sectors for which OSI values have been received, orare being utilized, and Decision, is the indicated power adjustment foreach terminal

This combined determination may then be used to adjust the power, block250. Various, exemplary approaches, are depicted and discussed withrespect to FIGS. 4A and 4B. However, other approaches and techniques maybe utilized.

In certain other aspects, additional functions may be utilized todetermine the power adjustment. For example, a terminal may find thesector with the highest channel gain and determine to the OSI value toutilize based upon whether strongest pilot transmissions and OSI valuewere received from that sector. For example, a terminal may make thisdetermination as follows:

$\begin{matrix}{{{OSI}\; 2{SequenceNum}} = \{ \begin{matrix}{{{{OSI}\; 2{SequenceNum}} + 1},} & \begin{matrix}{{{if}\mspace{14mu} {PilotPNCurrent}} =} \\{{PilotPNStrongest}\mspace{14mu} {and}} \\{{{OSI}\; 2{SequenceNum}} <} \\{{{OSI}\; 2{SequenceNumMax}} - 1} \\{{{and}\mspace{14mu} {OSIStrongest}} = 2}\end{matrix} \\{{{OSI}\; 2{SequenceMax}},} & \begin{matrix}{{{if}\mspace{14mu} {PilotPNCurrent}} =} \\{{PilotPNStrongest}\mspace{14mu} {and}} \\{{{OSI}\; 2{SequenceNum}} =} \\{{{OSI}\; 2{SequencNumMax}} - 1} \\{{{and}\mspace{14mu} {OSIStrongest}} = 2}\end{matrix} \\{2,} & \begin{matrix}{{{if}\mspace{14mu} {PilotPNCurrent}} \neq} \\{{PilotPNStrongest}\mspace{14mu} {and}} \\{{{OSI}\; 2{SequenceNum}} =} \\{{{OSI}\; 2{SequencNumMax}} - 1} \\{{{and}\mspace{14mu} {OSIStrongest}} = 2}\end{matrix} \\{1,} & {otherwise}\end{matrix} } & {{Eq}.\mspace{14mu} (13)} \\{{PilotPNStrongest} = \{ \begin{matrix}{{PilotPNCurrent},} & {{{if}\mspace{14mu} {OSIStrongest}} = 2} \\{{- 1},} & {otherwise}\end{matrix} } & {{Eq}.\mspace{14mu} (14)}\end{matrix}$

where OSI2SequenceNumMax is a predetermined value, PilotPNCurrent is thecurrent sector with the current largest channel gain, PilotPNStrongestis the prior sector with the largest channel gain, and OSI2SequenceNumis the number of consecutive times the current sector has sent thelargest OSI value for the terminal

The access terminal may then increase its ΔP(n) by a predetermined gainvalue if D_(W) is greater than or equal to a threshold, decrease itsΔP(n) by a predetermined gain, which may be the same or different thanthe gain used for increasing, or decrease its ΔP(n) by decrease gainmultiplied by the number of times the current sector has the largestchannel gain, if D_(W) is less than or equal to a second threshold.Furthermore, the ΔP(n) is generally limited to being between a minimumand maximum gain, which are predetermined parameters.

In certain aspects, the transmit power may be adjusted in adeterministic manner, a probabilistic manner, or some other manner. Fordeterministic adjustment, the transmit power is adjusted in apre-defined manner based on the pertinent parameters. For probabilisticadjustment, the transmit power has a certain probability of beingadjusted, with the probability being determined by the pertinentparameters. Exemplary deterministic and probabilistic adjustment schemesare described below.

FIG. 4A shows a flow diagram of a process 300 for adjusting transmitpower in a probabilistic manner. Process 300 may be performed by eachterminal and for each time interval in which an OSI value is transmittedfrom at least one neighbor sector. Initially, the terminal determinesthe combined OSI value o (block 312). The terminal then determineswhether the OSI value is ‘1’ or ‘0’, or a ‘2’, (block 314). In the casewhere it is ‘2’, the power would be decreased according to a maximumvalue.

If the OSI value is ‘1’, indicating a higher than nominal interferencelevel, then the terminal determines a probability for decreasing thetransmit power, Pr_(dn)(n) (block 322). Pr_(dn)(n) may be computed basedon the current transmit power delta, ΔP(n), and the channel gain ratiofor the strongest neighbor sector, r_(osib)(n), or a combined channelgain value as described below. The terminal then randomly selects avalue x between 0.0 and 1.0 (block 324). In particular, x is a randomvariable uniformly distributed between 0.0 and 1.0. If the randomlyselected value x is less than or equal to the probability Pr_(dn)(n), asdetermined in block 326, then the terminal decreases its transmit powerdelta by a ΔP_(dn) down step (block 328), as follows:

ΔP(n+1)=ΔP(n)−ΔP _(dn),  Eq (15)

Otherwise, if x is greater than Pr_(dn)(n), then the terminal maintainsthe transmit power delta at the current level (block 330). From blocks328 and 330, the process proceeds to block 342.

If the OSI value is ‘0’ in block 314, indicating a lower than nominalinterference level, then the terminal determines a probability forincreasing the transmit power, Pr_(up)(n), e.g., based on ΔP(n) andr_(osib)(n), as also described below (block 332). The terminal thenrandomly selects a value x between 0.0 and 1.0 (block 334). If therandomly selected value x is less than or equal to the probabilityPr_(up)(n), as determined in block 336, then the terminal increases itstransmit power delta by an ΔP_(up) up step (block 338), as follows:

ΔP(n+1)=ΔP(n)+ΔP _(up).  Eq (16)

The step sizes for ΔP_(up) and ΔP_(dn) may both be set to the samesuitable value (e.g., 0.25 dB, 0.5 dB, 1.0 dB, and so on). If x isgreater than Pr_(up)(n) in block 336, then the terminal maintains thetransmit power delta at the same level (block 330). From blocks 330 and338, the process proceeds to block 342.

In block 342, the terminal limits the transmit power delta ΔP(n+1), tobe within the allowable range [ΔP_(min), ΔP_(max)]. The terminal thencomputes the transmit power for the next time interval, P_(dch)(n+1),based on the transmit power delta, ΔP(n+1), and the reference powerlevel, P_(ref)(n+1), for the next time interval, as shown in equation(1) (block 344). The terminal then limits the transmit powerP_(dch)(n+1) to be within the maximum power level (block 346), asfollows:

$\begin{matrix}{{P_{dch}( {n + 1} )} = \{ \begin{matrix}{{P_{dch}( {n + 1} )},} & {{{{if}\mspace{14mu} {P_{dch}( {n + 1} )}} \leq P_{\max}},} \\{P_{\max},} & {{otherwise}.}\end{matrix} } & {{Eq}\mspace{14mu} (17)}\end{matrix}$

The terminal uses the transmit power P_(dch)(n+1) for the next timeinterval.

The probabilities Pr_(dn)(n) and Pr_(up)(n) may be a function of thetransmit power delta, ΔP(n), and the channel gain ratio for thestrongest neighbor sector, r_(osib)(n), or a combined channel gainvalue. Various functions may be used for Pr_(dn)(n) and Pr_(up)(n). Eachfunction may have a different impact on various power controlcharacteristics such as (1) the convergence rate of the transmit poweradjustment and (2) the distribution of transmit power deltas for theterminals in the system.

In an embodiment, the probabilities Pr_(dn)(n) and Pr_(up)(n) may bedefined as follows:

$\begin{matrix}{{{\Pr_{up}(n)} = {\max ( {\Pr_{{up},\min},{\lbrack {1 - {\Pr_{\Delta \; P}(n)}} \rbrack \cdot \lbrack {1 - {\Pr_{gain}(n)}} \rbrack}} )}},{and}} & {{Eq}\mspace{14mu} ( {18a} )} \\{{{\Pr_{dn}(n)} = {\max ( {\Pr_{{dn},\min},{{\Pr_{\Delta \; P}(n)} \cdot {\Pr_{gain}(n)}}} )}},{where}} & {{Eq}\mspace{14mu} ( {18b} )} \\{{{\Pr_{\Delta \; P}(n)} = \frac{{\min ( {{\Delta \; {P(n)}},{\Delta \; {\overset{\sim}{P}}_{\max}}} )} - {\Delta \; {\overset{\sim}{P}}_{\min}}}{{\Delta \; {\overset{\sim}{P}}_{\max}} - {\Delta \; {\overset{\sim}{P}}_{\min}}}},} & {{Eq}\mspace{14mu} ( {18c} )} \\{{{\Pr_{gain}(n)} = \frac{{\min ( {{r_{osib}(n)},r_{\max}} )} - r_{\min}}{r_{\max} - r_{\min}}},} & {{Eq}\mspace{14mu} ( {18d} )}\end{matrix}$

Pr_(ΔP)(n) is a probability related to the transmit power level;

Pr_(gain)(n) is a probability related to the channel gain ratio for thestrongest neighbor sector;

Δ{tilde over (P)}_(max), and Δ{tilde over (P)}_(min) r_(max), andr_(min) are normalizing constants selected to achieve the desired powercontrol characteristics;

Pr_(up,min) is a minimum probability for upward adjustment of transmitpower; and

Pr_(dn,min) is a minimum probability for downward adjustment of transmitpower.

For the embodiment shown by equation set (18), Pr_(dn)(n) and Pr_(up)(n)are joint probabilities determined by the transmit power level and thechannel gain ratio. The minimum probabilities Pr_(up,min) andPr_(dn,min) improve steady-state characteristics and promote somemovement for points in the extremes (e.g., very high or very low channelgain values). The probabilities Pr_(dn)(n) and Pr_(up)(n) derived asshown in equation set (15) conform to the general transmit poweradjustment rules discussed above, e.g. paragraph [0070]. Theprobabilities Pr_(dn)(n) and Pr_(up)(n) may also be derived with someother functions, and this is within the scope of the invention.

FIG. 4B shows a flow diagram of a process 400 for adjusting transmitpower in a deterministic manner. Process 400 may also be performed byeach terminal and for each time interval in which an OSI value istransmitted. The terminal processes the combined OSI value (block 412)and determines whether the OSI value is ‘1’ or ‘0’, or ‘2’, (block 414).If the OSI value is ‘1’, then the terminal determines the amount ofdecrease in transmit power, ΔP_(dn)(n+1), for the next time interval(block 422). The variable down step size may be determined based on thecurrent transmit power delta, ΔP(n), and the channel gain ratio,r_(osib)(n). The terminal then decreases the transmit power delta byΔP_(dn)(n+1) (block 424). Otherwise, if the OSI value is ‘0’, then theterminal determines the amount of increase in transmit power,ΔP_(up)(n+1), for the next time interval, e.g., based on ΔP(n) andr_(osib)(n) (block 432). The terminal then increases the transmit powerdelta by ΔP_(up)(n+1) (block 434). After blocks 424 and 434, theterminal limits the transmit power delta for the next time interval,ΔP(n+1), to be within the allowable range of [ΔP_(min), ΔP_(max)] (block442) and further computes and limits the transmit power for the nexttime interval to be within the maximum power level (blocks 444 and 446).

The variable step sizes ΔP_(dn)(n+1) and ΔP_(up)(n+1) and may bedetermined based on a predetermined function of ΔP(n) and r_(osib)(n),e.g., similar to the function expressed by equation set (15). Thevariable step sizes may be defined to be proportional to ΔP(n) andinversely proportional to r_(osib)(n). The adjustment probabilities andvariable step sizes may also be determined based on a look-up table ofdifferent probabilities and step size values for different ΔP(n) andr_(osib)(n) values, or by some other means.

FIGS. 4A and 4B show exemplary embodiments for adjusting transmit powerin a probabilistic and a deterministic manner, respectively. For theprobabilistic embodiment shown in FIG. 4A, the adjustment probability isdetermined based on the parameters ΔP(n) and r_(osib)(n), and fixed-sizeup and down steps are used for transmit power adjustment. For thedeterministic embodiment shown in FIG. 4B, the adjustment probability isfixed at 1.0, and the up and down step sizes are determined based on theparameters ΔP(n) and r_(osib)(n). Various modifications may also be madeto these embodiments. For example, variable up and down step sizes mayalso be used for the probabilistic embodiment. As another example,fixed-size up and down steps may be used for the deterministicembodiment.

The power delta ΔP(n) for the data channel may be adjusted based on theOSI value, the channel gain, the prior power delta ΔP(n−1), the range ofallowable power deltas, and the maximum power level for the terminal, asdescribed above. In general, the power delta ΔP(n) may be adjusted basedon any one or any combination of parameters. Other parameters that maybe used to adjust ΔP(n) include the current transmit power P_(dch)(n), apeak-to-average backoff factor ΔP_(bo), a “designated” set of sectorsthat may potentially observe high interference from the terminal, and soon. The peak-to-average backoff factor may be determined by the numberof sub-carriers used by the terminal for transmission, and a highervalue may be used for ΔP_(bo) if more sub-carriers are used fortransmission. The transmit power for the data channel may be constrainedto be less than Pmax minus this backoff factor, orP_(dch)(n)≦(P_(max)−ΔP_(bo))

The transmit power for the terminal may also be adjusted based on otherparameters, criteria, and information. The terminal may further adjustthe transmit power by different amounts and/or in different mannersbased on all of the information available for the sector(s) to beconsidered for transmit power adjustment.

FIG. 5 shows a power control mechanism 500 that may be used to adjustthe transmit power for a terminal 120 x in system 100. Terminal 120 xcommunicates with a serving sector 110 x and may cause interference toneighbor sectors 110 a through 110 m (albeit by different amounts).Power control mechanism 500 includes a reference loop 510 and a secondloop 520. Reference loop 510 operates between terminal 120 x and servingsector 110 x. Second loop 520 operates between terminal 120 x andneighbor sectors 110 a through 110 m and possibly serving sector 110 x.For simplicity, FIG. 5 shows only the portion of loops 510 and 520residing at terminal 120 x.

Reference loop 510 adjusts the transmit power for a control channel (orsome other traffic channel) and attempts to maintain the received SNRfor this control channel, as measured at serving sector 110 x, as closeas possible to a target SNR. For reference loop 510, serving sector 110x estimates the received SNR for the control channel, compares thereceived SNR against the target SNR, and generates transmit powercontrol (TPC) commands based on the comparison results, as describedbelow. Each TPC command may be either (1) an UP command to direct anincrease in transmit power for the control channel or (2) a DOWN commandto direct a decrease in transmit power. Serving sector 110 x transmitsthe TPC commands on the forward link (cloud 570) to terminal 120 x.

Terminal 120 x receives and processes the forward link transmission fromserving sector 110 x and provides “received” TPC commands to a TPCcommand processor 542. Each received TPC command is a noisy version of aTPC command transmitted by serving sector 110 x. Processor 542 detectseach received TPC command and obtains a “TPC decision”, which may be (1)an UP decision if the received TPC command is deemed to be an UP commandor (2) a DOWN decision if the received TPC command is deemed to be anDOWN command. A control channel transmit (TX) power adjustment unit 544adjusts the transmit power for the control channel, P_(cch)(n) based onthe TPC decisions from TPC command processor 542. For example, unit 544may increase P_(cch)(n) by a ΔP_(cch,up) up step for each UP decisionand decrease P_(cch)(n) by a ΔP_(cch,dn) down step for each DOWNdecision. A TX data processor/modulator 560 sets the transmit power forthe control channel to the P_(cch)(n) level indicated by unit 544. Thetransmission on the control channel is sent to serving sector 110 x.

Due to path loss, fading, and multipath effects on the reverse link(cloud 540), which typically vary over time and especially for a mobileterminal, the received SNR for the control channel continuallyfluctuates. Reference loop 510 attempts to maintain the received SNR ator near the target SNR in the presence of changes in the reverse linkchannel condition.

Second loop 520 adjusts the transmit power for a data channel (or someother traffic channel) such that a power level that is as high aspossible is used for the data channel while keeping inter-sector andintra-sector interference to within acceptable levels. For second loop520, an OSI value processor 552 receives and processes the OSI valuesbroadcast by neighbor sectors 110 a through 110 m and possibly servingsector 110 x. OSI value processor 552 provides detected OSI values fromthe sectors to a transmit power delta adjustment unit 556. A channelestimator 554 receives pilots from the serving and neighbor sectors,estimates the channel gain for each sector, and provide the estimatedchannel gains for all sectors to unit 556. Unit 556 determines thechannel gain ratios for the neighbor sectors and identifies thestrongest neighbor sector. Unit 556 further adjusts the transmit powerdelta ΔP(n) for the data channel based on either a combined OSI value,or a combined OSI value and the channel gain ratio for the strongestneighbor or a combined channel gain ratio, as described above. Unit 556may implement process 300 or 400 and may adjust ΔP(n) in a probabilisticor deterministic manner, or as otherwise discussed with respect to FIG.4A. In general, unit 556 may adjust the transmit power delta ΔP(n) basedon detected OSI values and/or other pertinent information for any numberof sectors, which may include the serving and/or neighbor sectors.

A data channel transmit power computation unit 558 receives the controlchannel transmit power, P_(cch)(n), which is used as the reference powerlevel, P_(ref)(n), and the transmit power delta, ΔP(n). Unit 558computes the transmit power P_(dch)(n) for the data channel based onP_(cch)(n) and ΔP(n). Unit 560 sets the transmit power for the datachannel to the P_(dch)(n) level indicated by unit 558. The transmissionon the data channel is sent to serving sector 110 x. The transmissionson the data and control channels may cause interference to neighborsectors 110 a through 110 m.

Each sector 110 receives transmissions from terminals on the reverselink, estimates the interference observed by that sector, compares themeasured interference against the nominal interference threshold, setsthe OSI value accordingly based on the comparison result, and broadcaststhe OSI value on the forward link.

Reference loop 510 and second loop 520 may operate concurrently but maybe updated at different rates, with loop 510 being a faster loop thanloop 520. The update rates for the two loops may be selected to achievethe desired power control performance. As an example, reference loop 510may be updated at a rate of, e.g., 150 times per second, and second loopmay be updated at a rate of, e.g., 10 to 20 times per second. Referenceloop 510 and second loop 520 may operate on transmissions sent on thecontrol channel and the data channel, respectively. The control and datachannels may be assigned different sub-carriers in each hop period, asshown in FIG. 2. In this case, reference loop 510 and second loop 520may operate simultaneously on transmissions sent on differentsub-carriers. The control channel may also be multiplexed with the datachannel (e.g., using TDM and/or CDM) and sent on the same sub-carriers.

FIG. 6 shows a power control mechanism 600 that may be used for thecontrol channel. Power control mechanism 600 (which may be used forreference loop 510 in FIG. 5) includes an inner loop 610, an outer loop620, and a third loop 630. Inner loop 610 attempts to maintain thereceived SNR for the control channel as close as possible to the targetSNR. For inner loop 610, an SNR estimator 642 at serving sector 110 xestimates the received SNR for the control channel and provides thereceived SNR to a TPC command generator 644. Generator 644 compares thereceived SNR against the target SNR and generates TPC commands based onthe comparison results. Serving sector 110 x transmits the TPC commandson the forward link (cloud 570) to terminal 120 x. Terminal 120 xreceives and processes the TPC commands from serving sector 110 x andadjusts the transmit power for the control channel, as described abovefor FIG. 5.

Data may be sent in blocks on the control channel, and each data blockmay be coded with a block code to obtain a corresponding codeword (orcoded data block). An error detection code may not be used for thecontrol channel. In this case, the serving sector may perform erasuredetection for each received codeword to determine whether the codewordis erased or non-erased. An erased codeword may be deemed to beunreliable and processed accordingly (e.g., discarded). The erasuredetection may be performed by computing a metric for each receivedcodeword, comparing the computed metric against an erasure threshold,and declaring the received codeword to be erased or non-erased based onthe comparison result.

Outer loop 620 adjusts the target SNR such that a target erasure rate,Pr_(erasure) is achieved for the control channel. The target erasurerate indicates a desired probability (e.g., 10%) of declaring a receivedcodeword as erased. A metric computation unit 652 computes the metricfor each received codeword. An erasure detector 654 performs erasuredetection for each received codeword based on its computed metric andthe erasure threshold and provides the status of the received codeword(erased or non-erased) to a target SNR adjustment unit 656. Unit 656then adjusts the target SNR for the control channel as follows:

$\begin{matrix}{{{SNR}_{target}( {k + 1} )} = \{ \begin{matrix}{{{{SNR}_{target}(k)} + {\Delta \; {SNR}_{up}}},} & {{foranerasedcodeword},} \\{{{{SNR}_{target}(k)} - {\Delta \; {SNR}_{dn}}},} & {{{foranon}\text{-}{erasedcodeword}},}\end{matrix} } & {{Eq}\mspace{14mu} (19)}\end{matrix}$

where SNR_(target)(k) is the target SNR for outer loop update intervalk;

ΔSNR_(up) is an up step size for the target SNR; and

ΔSNR_(dn) is a down step size for the target SNR.

The ΔSNR_(up) and ΔSNR_(dn), step sizes may be set based on thefollowing:

$\begin{matrix}{{\Delta \; {SNR}_{up}} = {\Delta \; {{SNR}_{dn} \cdot {( \frac{1 - \Pr_{erasure}}{\Pr_{erasure}} ).}}}} & {{Eq}\mspace{14mu} (20)}\end{matrix}$

Third loop 630 adjusts the erasure threshold such that a targetconditional error rate, Pr_(error) is achieved for the control channel.The target conditional error rate indicates a desired probability of areceived codeword being decoded in error when deemed to be non-erased. Asmall Pr_(error) (e.g., 1%) corresponds to high confidence in thedecoding results for non-erased codewords. Terminal 110 x and/or otherterminals in communication with serving sector 110 x may transmit knowncodewords on the control channel periodically or when triggered. Units652 and 654 perform erasure detection for each received known codewordin the same manner as for a received codeword. For each received knowncodeword deemed to be non-erased, a decoder 662 decodes the receivedknown codeword and determines whether the decoded data block is corrector in error. Decoder 662 provides the status of each received knowncodeword, which may be erased, “good”, or “bad”. A good codeword is areceived known codeword deemed to be non-erased and decoded correctly. Abad codeword is a received known codeword deemed to be non-erased butdecoded in error. An erasure threshold adjustment unit 664 adjusts theerasure threshold based on the status of each received known codeword,as follows

$\begin{matrix}{{{TH}_{erasure}( {l + 1} )} = \{ \begin{matrix}{{{{TH}_{erasure}(l)} + {\Delta \; {TH}_{up}}},} & {{foragoodcodeword},} \\{{{{TH}_{erasure}(l)} - {\Delta \; {TH}_{dn}}},} & {{forabadcodeword},{and}} \\{{{TH}_{erasure}(l)},} & {{foranerasedcodeword},}\end{matrix} } & {{Eq}\mspace{14mu} (21)}\end{matrix}$

where TH_(erasure)(l) is the erasure threshold for third loop updateinterval l;

ΔTH_(up) is an up step size for the erasure threshold; and

ΔTH_(dn) is a down step size for the erasure threshold.

Equation (21) assumes that a lower erasure threshold increases thelikelihood of a received codeword being declared erased.

The ΔTH_(up) and ΔTH_(dn) step sizes may be set based on the following:

$\begin{matrix}{{\Delta \; {TH}_{dn}} = {\Delta \; {{TH}_{up} \cdot {( \frac{1 - \Pr_{error}}{\Pr_{error}} ).}}}} & {{Eq}\mspace{14mu} (22)}\end{matrix}$

Inner loop 610, outer loop 620, and third loop 630 are typically updatedat different rates. Inner loop 610 is the fastest loop of the threeloops, and the transmit power for the control channel may be updated ata particular rate (e.g., 150 times per second). Outer loop 620 is thenext fastest loop, and the target SNR may be updated whenever a codewordis received on the control channel. Third loop 630 is the slowest loop,and the erasure threshold may be updated whenever a known codeword isreceived on the control channel. The update rates for the three loopsmay be selected to achieve the desired performance for erasure detectionand power control for the control channel. Power control mechanism 600is further described in commonly assigned U.S. patent application Ser.No. 10/890,717, entitled “Robust Erasure Detection andErasure-Rate-Based Closed Loop Power Control.”

For clarity, specific embodiments have been described above for variousaspects of power control. Numerous other embodiments may also be derivedbased on the description provided herein. Some examples are given below.

The same range of allowable transmit power deltas, [ΔP_(min), ΔP_(max)],may be used for all terminals in the system. Different ranges of[ΔP_(min), ΔP_(max)] may also be used for different terminals, e.g.,depending on their locations. For example, terminals with smallerchannel gain ratio for the strongest neighbor sectors may use a smallerrange of transmit power deltas (e.g., the same ΔP_(min) but a smallerΔP_(max)) than terminals located closer to the serving sectors.

The reference power level, P_(ref)(n), used to derive the data channeltransmit power, P_(dch)(n), may be set to the transmit power for anotherpower-controlled channel, as described above. The reference power levelmay also be obtained in other manners, e.g., estimated based on thechannel gain for the serving sector. The data channel transmit power mayalso be adjusted directly, instead of via the transmit power delta. Theserving sector may provide feedback to inform the terminal whether thedata channel transmit power is within an allowable range.

Each sector may broadcast its interference information to all terminals,if the interference observed by the sector is randomized, e.g., withfrequency hopping. If the sectors have more specific interferenceinformation, then the transmit powers of the terminals may be adjustedin a manner to take advantage of this information. For example, eachterminal may be assigned one or more specific sub-carriers for datatransmission (without frequency hopping). A sector may then observedifferent amounts of interference on different sub-carriers. Terminalscausing large amounts of interference may be specifically identifiedbased on their assigned sub-carriers, and the transmit powers of theseterminals may be reduced accordingly.

The supported data rate for each terminal is determined by the receivedSNR for the data channel. This received SNR, for the embodimentsdescribed above, is dependent on (1) the target SNR associated with thereference power level and (2) the transmit power delta, ΔP(n), used bythe terminal. The transmit power delta may be autonomously adjusted bythe terminal without any input from the serving sector, as describedabove. The terminal may send the transmit power delta, the received SNRfor the data channel, the supported data rate for the data channel, orequivalent information to the serving sector. The terminal may also sendthe maximum number of sub-carriers, N_(sb,max)(n), that the terminal cansupport at the current transmit power delta, the desired quality ofservice (QoS), the buffer size, and so on. To reduce the amount ofsignaling, the terminal may send ΔP(n) and N_(sb,max) (n) every fewupdate intervals, via in-band signaling on the data channel, and so on.

A scheduler at/for the serving sector may use all of the informationreported by the terminal to allocate resources to the terminal and toschedule the terminal for data transmission on the reverse link. Thescheduler may allocate N_(sb,max)(n) sub-carriers, less thanN_(sb,max)(n) sub carriers, or more than N_(sb,max)(n) sub-carriers tothe terminal. If the scheduler allocates more than N_(sb,max)(n)sub-carriers, then the terminal can scale down the transmit power deltaaccordingly. For example, if 2N_(sb,max)(n) sub-carriers are allocated,then ΔP(n) may be scaled down by a factor of two.

The power control may be performed by each terminal based on variouspieces of information the terminal obtains from its serving sector andneighbor sectors, as described above. The power control may also beperformed by each sector for all terminals in communication with thesector. For example, each sector may obtain an interference report(e.g., the OSI value) for each neighbor sector, e.g., via signalingbetween the sectors or transmissions from the terminals. Each sector mayalso obtain the channel gains determined by each terminal for theserving and neighbor sectors. Each sector may then compute the transmitpower delta for each terminal based on the interference reports and thechannel gains applicable for that terminal and may sent the transmitpower delta to the terminal. Each terminal may then adjust its transmitpower using the transmit power delta received from its serving sector.Alternatively, each sector may compute and send the transmit power foreach terminal. The availability of the transmit power deltas for allterminals in communication with each sector can expedite the schedulingfor the terminals.

The techniques described herein may be used for power control of varioustypes of traffic channels (e.g., data and control channels). Thesetechniques are also well suited for a hybrid automatic retransmission(H-ARQ) scheme. With H-ARQ, each coded packet is partitioned intomultiple (Nbl) subblocks, and one subblock is transmitted at a time forthe coded packet. As each subblock for a given coded packet is receivedvia the reverse link, the serving sector attempts to decode and recoverthe packet based on all subblocks received thus far for the packet. Theserving sector is able to recover the packet based on a partialtransmission because the subblocks contain redundant information that isuseful for decoding when the received SNR is low but may not be neededwhen the received SNR is high. The serving sector transmits anacknowledgment (ACK) if the packet is decoded correctly, and theterminal may terminate the transmission of the packet early uponreceiving the ACK.

With H-ARQ, each coded packet may be transmitted in a variable amount oftime until decoded correctly. A conventional power control mechanismthat adjusts the received SNR for the data channel based on packet errorrate (PER) would reduce the transmit power for the data channel to a lowlevel such that a target PER is achieved with all Nbl subblockstransmitted for each coded packet. This may severely reduce systemthroughput. The techniques described herein allow a high transmit powerlevel to be used even with variable duration transmission supported byH-ARQ.

FIG. 7 shows a block diagram of an embodiment of terminal 120 x, servingsector 110 x, and neighbor sector 110 a. On the reverse link, atterminal 120 x, a TX data processor 710 processes (e.g., codes,interleaves, and modulates) reverse link (RL) traffic data and providesmodulation symbols for the traffic data. TX data processor 710 alsoprocesses control data (e.g., a channel quality indicator) from acontroller 720 and provides modulation symbols for the control data. Amodulator (MOD) 712 processes the modulation symbols for the traffic andcontrol data and pilot symbols and provides a sequence of complex-valuedchips. The processing by TX data processor 710 and modulator 712 isdependent on the system. Modulator 712 performs OFDM modulation if thesystem utilizes OFDM. A transmitter unit (TMTR) 714 conditions (e.g.,converts to analog, amplifies, filters, and frequency upconverts) thesequence of chips and generates a reverse link signal, which is routedthrough a duplexer (D) 716 and transmitted via an antenna 718.

At serving sector 110 x, the reverse link signal from terminal 120 x isreceived by an antenna 752 x, routed through a duplexer 754 x, andprovided to a receiver unit (RCVR) 756 x. Receiver unit 756 x conditions(e.g., filters, amplifies, and frequency downconverts) the receivedsignal and further digitizes the conditioned signal to obtain a streamof data samples. A demodulator (DEMOD) 758 x processes the data samplesto obtain symbol estimates. A receive (RX) data processor 760 x thenprocesses (e.g., deinterleaves and decodes) the symbol estimates toobtain decoded data for terminal 120 x. RX data processor 760 x alsoperforms erasure detection and provides to a controller 770 x the statusof each received codeword used for power control. The processing bydemodulator 758 x and RX data processor 760 x is complementary to theprocessing performed by modulator 712 and TX data processor 710,respectively.

The processing for a forward link transmission may be performedsimilarly to that described above for the reverse link. The processingfor the transmissions on the forward and reverse links is typicallyspecified by the system.

For reverse link power control, at serving sector 110 x, an SNRestimator 774 x estimates the received SNR for terminal 120 x andprovides the received SNR to a TPC command (cmd) generator 776 x.Generator 776 x also receives the target SNR and generates TPC commandsfor terminal 120 x. The TPC commands are processed by a TX dataprocessor 782 x and a modulator 784 x, conditioned by a transmitter unit786 x, routed through duplexer 754 x, and transmitted via antenna 752 xto terminal 120 x. At neighbor sector 110 a, an interference estimator774 a estimates the interference observed by the sector and provides themeasured interference to an OSI value generator 776 a. Generator 776 aalso receives the nominal interference threshold and generates the OSIvalue for sector 110 a. The OSI value is processed and broadcast toterminals in the system. Generator 776 a may also generate a panic bitor some other type of interference report.

At terminal 120 x, the forward link signals from the serving andneighbor sectors are received by antenna 718. The received signal isrouted through duplexer 716, conditioned and digitized by a receiverunit 740, and processed by a demodulator 742 and an RX data processor744 to obtain received TPC commands and received OSI values. A channelestimator within demodulator 742 estimates the channel gain for eachsector. A TPC processor 724 detects the received TPC commands to obtainTPC decisions, which are used to update the transmit power for thecontrol channel. TPC processor 724 also adjusts the transmit power forthe data channel based on the received OSI values for neighbor sectors,the channel gains for the serving and neighbor sectors, and the transmitpowers for the data and control channels, as described above. TPCprocessor 724 (or controller 720) may implement process 300 in FIG. 4Aor process 400 in FIG. 4B. TPC processor 724 provides transmit poweradjustment controls for the control and data channels. Processor 710and/or modulator 712 receive the controls from TPC processor 724 andadjust the transmit powers for control and data channels.

Controllers 720, 770 x, and 770 a direct the operations of variousprocessing units within terminal 120 x and sector 110 x and 110 a,respectively. These controllers may also perform various functions forpower control for the reverse link. For example, controllers 720 and 770x may implement the processing units shown in FIGS. 5 and 6 for terminal120 x and sector 110 x, respectively and the processes described withrespect to FIGS. 3, 4A and 4B. Memory units 722, 772 x, and 772 a storedata and program codes for controllers 720, 770 x, and 770 a,respectively. A scheduler 780 x schedules terminals for datatransmission to/from serving sector 110 x.

The power control techniques described herein may be implemented byvarious means. For example, these techniques may be implemented inhardware, software, or a combination thereof. For a hardwareimplementation, the processing units used to perform power control maybe implemented within one or more application specific integratedcircuits (ASICs), digital signal processors (DSPs), digital signalprocessing devices (DSPDs), programmable logic devices (PLDs), fieldprogrammable gate arrays (FPGAs), processors, controllers,micro-controllers, microprocessors, other electronic units designed toperform the functions described herein, or a combination thereof.

For a software implementation, the power control techniques may beimplemented with modules (e.g., procedures, functions, and so on) thatperform the functions described herein. The software codes may be storedin a memory unit (e.g., memory unit 722 in FIG. 7) and executed by aprocessor (e.g., controller 720). The memory unit may be implementedwithin the processor or external to the processor, in which case it canbe communicatively coupled to the processor via various means as isknown in the art.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

1. A method of performing power control at a wireless terminal in awireless communication system, the method comprising: obtaining, for atleast two sectors, an indication of interference observed by the sector,each sector being a neighbor sector not designated to receive a datatransmission sent by the wireless terminal or a serving sectordesignated to receive the data transmission sent by the wirelessterminal; combining each indication of interference received from the atleast two sectors according to a weighting scheme based on a channelgain relationship for each sector with respect to a serving sector; andadjusting transmit power for the data transmission based on the combinedindications.
 2. The method of claim 1, wherein the indication comprisesa first bit that indicates whether the interference observed by thesector is above or below a first interference threshold.
 3. The methodof claim 2, wherein the indication further comprises a second bit thatindicates whether the interference observed by the sector exceeds asecond interference threshold that is higher than the first interferencethreshold.
 4. The method of claim 1, wherein the serving sectorcomprises a reverse link serving sector.
 5. The method of claim 1,further comprising determining a threshold value for each indication andwherein weighting comprises weighting each threshold value according tothe channel gain relationship.
 6. The method of claim 1, wherein channelgains for each of the at least two sectors and the serving sectors areestimated based on pilots received from the sectors, respectively. 7.The method of claim 1, wherein adjusting the transmit power comprisesadjusting based on the combined indication and a probability.
 8. Themethod of claim 7, further comprising determining the probability foradjusting the transmit power upward or downward based on the channelgain relationships for each of the at least two sectors.
 9. The methodof claim 8, wherein the probability is determined further based on acurrent level of the transmit power for the data transmission.
 10. Themethod of claim 8, wherein the transmit power is adjusted in afixed-size step and in accordance with the determined probability. 11.The method of claim 1, further comprising determining a step size foradjusting the transmit power based on the estimated channel gainrelationships and wherein adjusting comprises adjusting based on thecombined indications and the step size.
 12. The method of claim 11,wherein the step size is determined further based on a current level ofthe transmit power for the data transmission.
 13. An apparatus operableto perform power control at a wireless terminal in a wirelesscommunication system, comprising: a processor configured to obtain, forat least two sectors, an indication of interference observed by thesector, each sector being a neighbor sector not designated to receive adata transmission sent by the wireless terminal or a serving sectordesignated to receive the data transmission sent by the wirelessterminal, to combine each indication of interference received from theat least two sectors according to a weighting scheme based on a channelgain relationship for each sector with respect to a serving sector, andto adjust a transmit power for data transmission based on the combinedindication of interference received from the at least two sectors; and amemory coupled with the processor.
 14. The apparatus of claim 13,wherein the indication comprises a first bit that indicates whether theinterference observed by the sector is above or below a firstinterference threshold.
 15. The apparatus of claim 14, wherein theindication further comprises a second bit that indicates whether theinterference observed by the sector exceeds a second interferencethreshold that is higher than the first interference threshold.
 16. Theapparatus of claim 13, wherein the serving sector comprise a reverselink serving sector.
 17. The apparatus of claim 13, wherein theprocessor is configured to determine a threshold value for eachindication and weight each threshold value according to the channel gainrelationship.
 18. The apparatus of claim 13, wherein the processor isconfigured to adjust the transmit power based on the combined indicationand a probability.
 19. The apparatus of claim 18, wherein the processoris configured to determine the probability, for adjusting the transmitpower upward or downward, based on the channel gain relationships foreach of the at least two sectors.
 20. The apparatus of claim 19, whereinthe processor is configured to determine the probability based on acurrent level of the transmit power for the data transmission.
 21. Theapparatus of claim 19, wherein the processor is configured to adjust thetransmit power in a fixed-size step and in accordance with thedetermined probability.
 22. The apparatus of claim 13, wherein theprocessor is configured to determine a step size and to adjust based onthe combined indications and the step size.
 23. The apparatus of claim22, wherein the processor is configured to determine the step size basedon a current level of the transmit power for the data transmission. 24.An apparatus operable to perform power control at a wireless terminal ina wireless communication system, comprising: means for obtaining, for atleast two sectors, an indication of interference observed by the sector,each sector being a neighbor sector not designated to receive a datatransmission sent by the wireless terminal or a serving sectordesignated to receive the data transmission sent by the wirelessterminal; means for combining each indication of interference receivedfrom the at least two sectors according to a weighting scheme based on achannel gain relationship for each sector with respect to a servingsector; and means for adjusting transmit power for the data transmissionbased on the combined indications.
 25. The apparatus of claim 24,wherein the indication comprises a first bit that indicates whether theinterference observed by the sector is above or below a firstinterference threshold.
 26. The apparatus of claim 25, wherein theindication further comprises a second bit that indicates whether theinterference observed by the sector exceeds a second interferencethreshold that is higher than the first interference threshold.
 27. Theapparatus of claim 24, further comprises means for determining athreshold value for each indication and wherein the means for weightingcomprises means for weighting each threshold value according to thechannel gain relationship.
 28. The apparatus of claim 24, furthercomprising means for estimating channel gains based on pilots received.29. The apparatus of claim 24, wherein the means for adjusting thetransmit power comprises means for adjusting based on the combinedindication and a probability.
 30. The apparatus of claim 24, wherein themeans for adjusting the transmit power comprises means for determining astep size for adjusting the transmit power based on the estimatedchannel gain relationships and adjusting based on the combinedindications and the step size.
 31. A computer readable storage mediumcomprising code, which, when executed by a processor, direct theprocessor to perform power control at a wireless terminal in a wirelesscommunication system, the computer readable storage medium comprising:code for obtaining, for at least two sectors, an indication ofinterference observed by the sector, each sector being a neighbor sectornot designated to receive a data transmission sent by the wirelessterminal or a serving sector designated to receive the data transmissionsent by the wireless terminal; code for combining each indication ofinterference received from the at least two sectors according to aweighting scheme based on a channel gain relationship for each sectorwith respect to a serving sector; and code for adjusting transmit powerfor the data transmission based on the combined indications.
 32. Thecomputer readable storage medium of claim 31, wherein the indicationcomprises a first bit that indicates whether the interference observedby the sector is above or below a first interference threshold.
 33. Thecomputer readable storage medium of claim 32, wherein the indicationfurther comprises a second bit that indicates whether the interferenceobserved by the sector exceeds a second interference threshold that ishigher than the first interference threshold.
 34. The computer readablestorage medium of claim 31, further comprising code for determining athreshold value for each indication and wherein the code for weightingcomprises code for weighting each threshold value according to thechannel gain relationship.
 35. The computer readable storage medium ofclaim 31, further comprising code for estimating channel gains based onpilots received.
 36. The computer readable storage medium of claim 31,wherein the code for adjusting the transmit power comprises code foradjusting based on the combined indication and a probability.
 37. Thecomputer readable storage medium of claim 31, wherein the code foradjusting the transmit power comprises code for determining a step sizefor adjusting the transmit power based on the estimated channel gainrelationships and adjusting based on the combined indications and thestep size.